1. Field of the Invention
The present invention relates to aqueous surfactant compositions having elevated viscosity which have a low foreign electrolyte content. In particular, the present invention relating to aqueous surfactant compositions may be used in cosmetic compositions such as, for example, shampoos, bath and shower preparations, hand-wash pastes and lotions.
2. Background Information
High viscosity aqueous surfactants are desirable in many cases. They are easier to handle and can be metered simply. If the composition contains a second phase (solid or liquid), the elevated viscosity also imparts higher storage stability. Moreover, higher viscosity aqueous surfactant compositions are of interest for marketing reasons, since the consumer frequently associates preparations of low viscosity with a low concentration of the active compound.
Aqueous surfactant compositions usually contain anionic surfactants, such as, fatty alcohol ether-sulphates, fatty alcohol sulphates, fatty alcohol sulphosuccinates, alkane sulphonates or ethercarboxylic acids, as the main component, and are sometimes found in combination with betaines, ampholytic compounds or fatty acid alkanolamides, for example. In the case of fatty alcohol ether sulphates and fatty alcohol sulphates, a simple increase in the viscosity can be obtained by adding water-soluble inorganic salts (foreign electrolytes), such as NaCl, NH.sub.4 Cl and Na.sub.2 SO.sub.4, but the amount of electrolytes which needs to be added are fairly high, which is undesirable, since in higher concentrations the electrolytes may be irritating. Many other surfactants which are of particular interest for use in the above compositions because of their high tolerance by the skin or mucous membranes, either cannot be thickened or can be thickened only to a small extent by adding foreign electrolytes.
Fatty acid alkanolamides can be used as a consistency regulator, but are considered undesirable since the low content of free alkanolamine can give rise to the formation of nitrosamine as a byproduct. Nitrogen-free additives are therefore preferred, instead of the alkanolamides (compare H. Hensen et al., 2nd World Surfactant Congress, Paris 1988, Vol. II, pages 378 ff.).
Saturated or unsaturated fatty alcohol oxethylates having low degrees of ethoxylation, preferably containing about 2.5 mol EO/mol (A. Behler et al., Seifen, Ole, Fette, Wachse 116, 60 (1990)) constitute one solution to this problem, but are also considered to be somewhat disadvantageous since they possess very high fat-dissolving power and their limited solubility in water reduces their foamability. Moreover, their effectiveness in compositions simulating those used in practice, that is to say, with realistic amount of the electrolyte is restricted to sulphate surfactants (A. Behler et al. loc. cit.).
Thickeners for aqueous solutions, which are effective irrespective of the surfactant type, belong to the group of water-soluble polymers. Suitable additives here are cellulose derivatives and xanthans. Polyethylene glycol derivatives (German Patent 3,140,160), polyol monoethers (European Patent 0,303,187), fatty acid-esterified polyoxyalkylene ethers of glycerol or propane-1,2-diol (German Patent 3,239,564) or other polyhydric alcohols (German Patent 3,843,224), and alkylpolyethylene glycol ether fatty acid esters (German Patent 3,541,813), for example, have also been disclosed. The thickening action of these additives is presumably due to a highly hydrated lattice build up, resulting in the partial immobilization of water. In this case, a certain synergism is also sometimes observed between the surfactant and the polymer, but the polymer concentration required to obtain the desired viscosity value is so high that the final aqueous solution is relatively expensive. Also the use of polymers such as those based on polyethylene glycol is questionable from an ecological point of view since said polymers, are not adequately biodegradable.
In addition, there are processing disadvantages and sometimes unsatisfactory rheological characteristics that arise in the case of high polymer concentrations. Therefore, polymers should only be used in very low concentrations in cosmetic compositions.
A thickener mixture is described in German Patent 3,843,224 which utilizes the surfactant/polymer combination in a targeted manner for surfactant compositions. The patent claims a mixture of a nonionic surfactant (HLB value of 4 to 11) with a polymer in a weight ratio of 10:1 to 1:10 as a thickener for other surfactant mixtures. Typical nonionic surfactants disclosed are oxethylates having low degrees of EO, that is to say, precisely those surfactants which, although they have a considerable thickening effect are questionable both toxicologically in respect to their tolerance by the skin and mucous membranes, and ecologically (compare P. Schobel et al., Tenside Surfactants Detergents 25.2 (1988), S. Matsumura JAOCS 67, p. 996 (1990)).
Thus, the need exists for a thickener mixture that would increase the viscosity of an aqueous, cosmetic surfactant composition without the disadvantages described above. The present invention provides for such a composition that is toxicologically, ecologically and economically superior to any previously known surfactant composition.
The present invention has achieved this objective by providing an aqueous surfactant composition that comprises a mixture of surfactant, polymer, and optionally an electrolyte, as a thickener to a surfactant base composition.